Protective coating and method of producing the same



Patented Jan. 24, 1933 UNITED STATES PATENT oEEicE JOHN A. STAPLES, OF PLAINI'IELD, NEW JERSEY, ASSIGNOB TO NATIONAL ELECTRIC PRODUCTS CORPORATION, OF NEW YORK, N. Y A CORPORATION OI DELAWARE PROTECTIVE COATING AND METHOD OF PRODUCING THE sum Drawing.

' ferrous articles with such 'coatings.

The invention has for one of objects the provision of an improved rotective coating for resisting corrosion w ich coating improves the appearance of the product, and which coating is adapted for use upon non: ferrous articles such as tubes, plates or other parts formed of brass and likealloys.

A further object of the present invention resides in the provision of a novel method ofobtaining the desired coating.

A further object of the present invention resides in the provision of a method of coating brass alloys and like nonferrous materials by the use of which uniformity of product may be secured at less expense than before and with less loss. a

A further object of the resent invention resides in the provision o coatin process which will enable coatings to be app ied u on articles which heretofore were not rea ily capable of receiving coatings on account of their composition or proportions and furthermore to provide a coating process which will enable proper coatings to be applied while at the same time, preventing over annealin of the alloy during the formation of the coatmg. Further and other objects of the present invention will be hereinafter set forth in the specification and claims. A careful study of the causes of failure of brass alloy tubes in heat exchangers,- conde'nsers, etc., shows that a large percentage of failures are caused by corrosion of the surfaces subjected to the conveyed fluids. To protect these surfaces with a non-corrosive substance would be ideal and very desirable for lengthening the usable tube life Attem ts have been made to accomplish these resu ts heretofore by the use of various nonmetallic compounds, but these results have not met with much success because of the lack of uniformity of the film and frequently the Application fled September 88, 1930. Serial No. 485,314.

compounds have been of such nature as to cause considerable loss in heat transfer.

It has accordingly been appreciated that an ideal protective coating would be a metal-. lic oxide coating. Such a coating irrgeneral is-less soluble in solutions than the correspondin metal or allo It is also apparent that wit brass alloy t e natural choice of a protectivecoating should fall on cupric oxide as copper is the predominating metal in the alloy. Such a coating should be an integral part of the tube and would allow the maximum amount of heat transfer. It may be pointed out that a rolonged exposure of a ,brass alloy to the e ements produces a compact film of oxides on the alloys which is known as weathering. The weatheringveffect may be greatly accelerated by chemical action. Also heat applied to the brass alloy in the presence of air will produce oxidization of the surfaces and result in a film of oxide which in reality can be considered an 'intergral part of metalin the tube. This From the above equations it will be seen that the presence of msujlicient oxygen pro duces the relatively unstable red cuprous oxide which on further oxidation becomes the black cupric oxide. The more desirable oxide coating would be the black cupric oxide coating which it has been shown can be produced by annealing brass in the presence of sufficient oxygen. This process is sometimes attempted in commercial practice by leaving on the tube or other part the 'film of oxide produced by the final annealing o'perationinstead of cleaning off the film with acids. Attempts have been made heretofore to accelerate this action by dippin the tubes in various compounds. It has 11 found, however, that it is impossible with the ordinary annealing furnaces to secure a uniform of oxide; in fact portions of the tube would not be covered at all. In other cases while the exterior of the tubes were properly coated with the desired black oxide coating the interior of the tubes or portions thereof were coated with a cuprous oxide coating, which coating did not provide the desired proper protection.

To attain improved degrees of perfection in the applied coatings, special furnaces have been provided whereby continuous uniform excess of oxygen. However, variations in the films occurredeven when the special furnaces, were employed and even when the compounds which were heretofore available to provide films, were used.

The application of black oxide coatings to brass tubes or like non-ferrous alloy parts also in practice presents other difiiculties. Such material for proper heat treatment requires the subjectingof the material to heats of varying degrees of temperature depending upon the initial hardness of the material. With higher degrees of hardness after drawing or working operations, relatively lower temperatures are required for complete recrystallization, and with relatively lower degrees of hardness relatively higher temperatures are required. Time factors are also involved. If a certain time of heat treatment is exceeded at a given temperature over annealing will occurand the grain structure of the alloy will become too large. Formation of a black oxide requires a given temperature and the time factor to obtain a proper coating will vary with a variation of a mass of the article to be coated, because the high conductivity of the material will carry heat away from the surface to be coated so that it will not oxidize as properly or as readily as it should to provide a proper coating. Furthermore, variation in the zinc content of the. alloy also necessitates a variation in the time factor for producing a proper coating because with material having relatively greater zinc content more material has to be removed to leave suflicient oxide to provide the proper coating; To properly oxidize the material, the more time that is used for the heat treatment, the better the coating will be within certain limits. However, the time of treatment for securing, the proper and desired oxide coatings will frequently exceed the time permitted for heat treatment to obtain proper grain construction in the alloy.

To provide a proper and relatively uniform black oxide coating under the foregoing conditions which are to a certain extent antagonistic to each other, requires a modification to the previously used process which will now be described. In lieu of first annealing the article, then immersing it in a bath of drawing compound comprised of tallow, soap, water and oil, then drawing the article and subsequently annealing the tube 65 orother part in the presence of an excess of heat could be applied in the presence of an oxygen, the process is modified by" adding to the usual drawing compound a quantity of copper carbonate in powdered form. Since this material is insoluble the drawing compound is mechanically agitated so as to keep the copper carbonate in suspension so as to uniformly coatthe tubes or other parts when they are dipped in the compound preparatory to drawing. In practice the quantity of copper carbonate may be widely varied, but suitable proportions in practice have been i found to comprise about five pounds of powdered copper carbonate for each fifty gallons of drawing, solution or compound.

After the copper carbonate has been applied to the tubes in the foregoing manner with the drawing compound, the subsequent drawing operation squeezes the carbonate into the surface of the tube and produces a film of carbonate. During the subsequent annealing step of the process the heatand excess of oxygen converts the copper carbonate to black cupric oxide and at the same time cupric oxide is formed from the copper of the tube itself by the oxidizing action of the applied oxygen from the air. Subsequent drawing and annealing of the black oxidecoated tube will further tend to impregnate the oxide into the surface and makeit more uniform in texture. By the use of copper carbonate in the manner described a part of the ultimate black oxide is derived from the copper in the carbonate as the same is oxidized and another portion of the oxide film is derived from the material from which the tube is composed. By utilizing copper carbonate in this manner coatings of uniform texture are produced. Furthermore thetime factors for providing a-coating are such that over annealing of the alloy does not occur and furthermore proper coatings can be provided on parts of such proportions which heretoforev did not lend themselves to coating with attendant proper heat treatment. Also coatings may be produced upon alloys which have so high a zinc content as not to readily lend themselves heretofore to the producing of black oxide coatings thereon with attendant proper heat treatment of the alloy. v

The established practice in the art is to pickle with diluted sulphuric acid after every annealing operation. Such pickling step is J necessary with non-uniform and irregular oxide formations upon the tube. However, according to the present process with some alloys the practice can be adopted of complete 1y eliminating the customary pickling operations after annealing. In fact some parts may be made with no pickling operations whatsoever. Obviously a number of drawing and oxide forming operations may be employed.

In other cases with different alloys, pickling steps after annealing are retained except for the final heat treatment and coating formation step.

What I claim is:

1. The method of improving the character of a protective coating upon a copper or copper alloy article such as a brass alloy which comprises creating the coating in part by the oxidation of the surface of the alloy itself and in part by forcing into the surface to be oxidized a substance comprising copper carbonate, and thereafter converting said copper carbonate into oxide form which joins and merges with oxide formed from and upon the surface of the alloy.

2. An article formed of a copper or copper alloy with a protective oxidized coating thereon with uniform characteristics provided by one oxide formed from and upon the surface of the alloy article itself and from another material which is exteriorly applied and pressed into the surface oxide and afterwards converted into the oxide state and merged with the similar surface oxide derived from the article itself.

3. A copper or copper alloy article provided with a corrosion resisting protective coating with characteristics of uniformity over the surface of the article provided by a dual cupric oxide coating, one oxide coating being formed from and upon the surface of the alloyed article itself and the other oxidized coating being exteriorly applied and created over the first mentioned oxidized coating, said dual oxide coating being firmly impregnated into the surface of the article.

4. A copper or copper alloy article with a protective coating thereon formed of merged like oxides, one derived under heat from the surface of the alloy itself and the other derived from an added material comprising copper carbonate to cupric oxide by the action of heat in the presence of an excess of oxygen.

In testimony whereof I hereto aflix my signature.

. JOHN A. STAPLES.

copper carbonate which is converted'by heat into the oxide state and into a similar oxide as that formed from the alloy article, said coating being firmly impregnated into the surface of the article.

5. The method of improving the character of a protective coating upon a copper or copper alloy part which is drawn and heat treated, which comprises they step of utilizing copper carbonate in the drawing compound and in subsequently creating a part of the oxide coating by a reduction of the copper carbonate to cupric oxide by the application of heat in the presence of an excess of oxygen.

6. The method of providing a coating upon a copper or copper alloy part such as a tube or the like which comprises forming one oxide upon the surface to be protected by oxidation of the material of the part itself and forming and impregnating into the surface of the part an additional quantity of the same 

